The induction of hemoglobin and globin mRNA by the thymidine analog, bromodoxyuridine (BrdU) in Friend virus transformed mouse leukemic cells is being examined. This induction is observed in cell lines in which BrdU can also be shown to interfere with the induction of hemoglobin and globin mRNA by dimethylsulfoxide (DMSO). Clones of Friend cells which are not inducible by DMSO will be examined for their response to BrdU. We will determine if the analogue must be incorporated into DNA by examining the response of thymidine kinase deficient Friend cell lines to BrdU. Studies directed towards examining the importance of transcriptional and post transcriptional regulating mechanisms for controlling gene expression in different differentiated cell types are also being carried out. These experiments involve the preparation of radioactive complementary DNA (cDNA) probes for the quantitation of specific RNA sequences in the nuclear and cytoplasmic fractions of cells manifesting different differentiated phenotype by DNA-RNA hybridization techniques. The cDNA probes currently being assessed are derived from the most abundant mRNA sequences in the free and membrane bound polysomes of rat liver.